>become allies
They already WERE allies, just to the ruling dynasty
That's actually how the war started, to restore the previous dynasty to the Roman throne…
>stop Arabs
Lol, with what army? The reason Arabs succeeded is because they were a huge chunk of the mercenaries on both sides running around Mesopotamia and the Levant after the Romans and Persians slaughtered two generations of each others fighting age males. They simply demanded their due after the war.
Islam came much later btw under Uthman, the Arabs at this time were Nestorians/pagans.